前言

我在[003]AIDL是什么中介绍的AIDL，但是好像还有朋友不明白问我，那我就来写一个终极版的文章，让你十分钟彻底明白AIDL，以下代码全为手写。

目标

Server进程注册一个Binder服务到SM，该Binder服务提供两个接口：add和minus
Client 进程通过SM获得Binder服务的代理类BinderProxy，并调用两个接口add，minus

1 没有AIDL的世界

1.1 Server进程

    class CommandBinder extends Binder {
        @Override
        protected boolean onTransact(int code, @NonNull Parcel data, @Nullable Parcel reply, int flags) throws RemoteException {
            if (code == 1) {
                int a = data.readInt();
                int b = data.readInt();
                reply.writeInt(a + b);
                return true;
            } else if (code == 2) {
                int a = data.readInt();
                int b = data.readInt();
                reply.writeInt(a + b);
                return true;
            }
            return super.onTransact(code, data, reply, flags);
        }
    }

    void main() {
        //注册binder服务
        ServiceManager.addService("command", new CommandBinder());
    }

1.2 Client进程

    int add(int a, int b) {
        int code = 1;
        BinderProxy mClient = ServiceManager.getService("command");
        Parcel data = Parcel.obtain();
        data.writeInt(a);
        data.writeInt(b);
        Parcel reply = Parcel.obtain();
        mClient.transact(code, data, reply, 0);//flag设置成0,调用这个方法会跨进程调用Binder服务类中onTransact方法
        int result = reply.readInt();
        return result;
    }

    int minus(int a, int b) {
        int code = 2;
        BinderProxy mClient = ServiceManager.getService("command");
        Parcel data = Parcel.obtain();
        data.writeInt(a);
        data.writeInt(b);
        Parcel reply = Parcel.obtain();
        mClient.transact(code, data, reply, 0);//flag设置成0,调用这个方法会跨进程调用Binder服务类中onTransact方法
        int result = reply.readInt();
        return result;
    }

2 有AIDL的世界

AIDL文件

    //aidl
    interface ICommand {
        int add(int a, int b);
        int minus(int a, int b);
    }

2.1 Server进程

与1.1中代码比较，我们发现，我们不是直接继承Binder，而是继承aidl文件自动生成ICommandBinder，我们只需专注写add和minus接口的实现，不需要在Binder类中的onTransact写一堆if else代码和Parcel.read write.

    class ADILCommandBinder extends ICommandBinder {
        @Override
        public int add(int a, int b) {
            return a + b;
        }

        @Override
        public int minus(int a, int b) {
            return a - b;
        }
    }

    void main() {
        //注册binder服务
        ServiceManager.addService("command", new ADILCommandBinder());
    }

2.2 Client进程

与1.2中代码比较，我们发现，我们不需要写Parcel.read write和BinderProxy.transact，而是用BinderProxy对象构造一个CommandBinderProxy对象，然后直接调用CommandBinderProxy的add和minus

    int add(int a, int b) {
        BinderProxy mClient = ServiceManager.getService("command");
        return new CommandBinderProxy(mClient).add(a, b);
    }

    int minus(int a, int b) {
        BinderProxy mClient = ServiceManager.getService("command");
        return new CommandBinderProxy(mClient).minus(a, b);
    }

3 ADIL自动生成了什么代码

生成了三个部分：ICommand，ICommandBinder，CommandBinderProxy

3.1 ICommand

看起来和aidl文件差不多

    interface ICommand {
        int add(int a, int b);
        int minus(int a, int b);
    }

3.2 ICommandBinder

继承于Binder，并实现ICommand，然后在onTransact方法中调用还没有实现的add和minus方法

   abstract public class ICommandBinder extends Binder implements ICommand {
        int add = 1;
        int minus = 2;

        @Override
        protected boolean onTransact(int code, @NonNull Parcel data, @Nullable Parcel reply, int flags) throws RemoteException {

            if (code == 1) {
                int a = data.readInt();
                int b = data.readInt();
                reply.writeInt(add(a, b));
                return true;
            } else if (code == 2) {
                int a = data.readInt();
                int b = data.readInt();
                reply.writeInt(minus(a, b));
                return true;
            }

            return super.onTransact(code, data, reply, flags);
        }

        abstract public int add(int a, int b);

        abstract public int minus(int a, int b);

    }

3.3 CommandBinderProxy

对BinderProxy进行封装，把调用CommandBinderProxy的add转化成Parcel.write,read和BinderProxy.transact代码。

    public class CommandBinderProxy implements ICommand {

        private BinderProxy remote;

        int add = 1;
        int minus = 2;

        public CommandBinderProxy(BinderProxy remote) {
            this.remote = remote;
        }

        @Override
        public int add(int a, int b) {
            Parcel data = Parcel.obtain();
            data.writeInt(a);
            data.writeInt(b);
            Parcel reply = Parcel.obtain();
            remote.transact(add, data, reply, 0);
            int result = reply.readInt();
            return result;
        }

        @Override
        public int minus(int a, int b) {
            int code = 2;
            Parcel data = Parcel.obtain();
            data.writeInt(a);
            data.writeInt(b);
            Parcel reply = Parcel.obtain();
            remote.transact(code, data, reply, 0);
            int result = reply.readInt();
            return result;
        }
    }